The vast majority of commercial radiation curable compositions contain low molecular weight photo-initiators and co-initiators. When low molecular weight products are not built into the polymer network, they are prone to diffuse out of the cured composition and can readily be extracted. When radiation curable compositions are used for food packaging or dental applications, the amount of extractable residues is a critical issue and needs to be minimized.
Especially Norrish type II initiators are a point of concern regarding extractable residues. Norrish type II photo-initiators always require a co-initiator. A co-initiator or synergist is basically a molecule capable of transferring a hydrogen atom to the excited state of the Norrish type II initiator. Aliphatic tertiary amines, aromatic amines and thiols are preferred examples of co-initiators. After transfer of a hydrogen atom to the Norrish type II initiator, the radical generated on the synergist initiates the polymerization. Theoretically the co-initiator is built into the polymer network. However, it is highly unlikely that both the hydrogen transfer and the initiation reaction yields are a hundred percent. Side reactions are likely to occur leaving unreacted synergist and side products in the composition. In food packaging printed upon with such a radiation curable composition, these low molecular weight residues remain mobile and if toxic will cause health risks upon being extracted into the food.
One approach in solving these problems is to design co-initiators and Norrish type II initiators with a higher molecular weight.
JP 2000086713 (TOYO INK) discloses the use of the reaction product of an unsaturated monomer bearing (meth)acryloyl groups or vinyl ether groups having an number average molecular weight of more than 500 with a primary or secondary amine as co-initiator in radiation curable compositions. However, using this approach only co-initiators with low functionality can be obtained.
EP 434098 A (UNION CARBIDE) discloses the use of amino terminated polyoxyalkylenes as co-initiators in radiation curable compositions. The claimed polyoxyalkylenes also have a low functionality, requiring the use of large amounts of unreactive polymer in the matrix compared to low molecular weight co-initiators.
WO 0222700 (PERSTORP SPECIALTY CHEM) discloses a radiation curable dendritic oligomer or polymer, characterised in that the radiation curable dendritic oligomer or polymer normally has at least one terminal group of Formula (A):
and normally at least one terminal group of Formula (B):
wherein R1 and R2 individually are hydrogen or methyl and wherein R3 and R4 individually are alkyl, aryl, alkylaryl, arylalkyl, alkylalkoxy, arylalkoxy, said alkyl and/or said aryl optionally having one or more hydroxyl groups. The dendritic polymers are claimed to be of particular interest for curing under air compared to conventional curable dendritic oligomers. However, these oligomeric co-initiators tend to lose their effectiveness when coupled to a polymer, which does not contain acrylates, as stated in DAVIDSON, Stephen R. Exploring the Science Technology and Applications of UV and EB-curing. LONDON, UK: SITA Technology Ltd, 1999. p. 141. and DAVIDSON, Stephen R., et al. Type II polymeric photoinitiators (polyetherimides) with built-in amine synergist. Journal of Photochemistry and Photobiology, A: Chemistry. 1995, vol. 91, no. 2, p. 153-163.
Polymeric initiators have been disclosed in CRIVELLO, J. V., et al. Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation. Surface Coatings Technology. 1998, vol. III, p. 208-224. and CORRALES, T., et al. Free radical macrophotoinitiators: an overview on recent advances. Journal of Photochemistry and Photobiology A: Chemistry. 2003, vol. 159, no. 2, p. 103-114. All the disclosed polymeric initiators have a conventional linear molecular geometry. The solution viscosity of a radiation curable composition is influenced significantly using these polymeric initiators.
WO 03033452 (COATES BROTHERS PLC) discloses multifunctional benzophenone initiators having the following general structure:
where n is a number from 1 to 6; R3 is hydrogen, methyl or ethyl; A represents a group of formula —[O(CHR2CHR1)a]y—, —[O(CH2)bCO]y, or —[O(CH2CO](y-1)—[O(CHR2CHR1)a]— (where one of R1 and R2 is hydrogen and the other is hydrogen, methyl or ethyl); a is from 1 to 2; b is from 4 to 5; y is from 3 to 10; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxyl groups; and x is greater than 1 but no greater than the number of available hydroxyl groups in Q.
WO 03033492 (COATES BROTHERS PLC) discloses similar polymeric initiators having the following structure:
where n is a number from 1 to 6; R3 is hydrogen, methyl or ethyl; A represents a group of formula —[O(CHR2CHR1)a]y—, —[O(CH2)bCO]y, or —[O(CH2CO](y-1)—[O(CHR2CHR1)a]— (where one of R1 and R2 is hydrogen and the other is hydrogen, methyl or ethyl); a is from 1 to 2; b is from 4 to 5; y is from 3 to 10; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxyl groups; and x is greater than 1 but no greater than the number of available hydroxyl groups in Q.
Both WO 03033452 (COATES BROTHERS PLC) and WO 03033492 (COATES BROTHERS PLC) teach that the molecular weight of the multifunctional initiators is most preferably lower than 800, since higher molecular weights cause an unwanted increase in the viscosity of the radiation curable formulation. This limits the functionality of the multifunctional initiator and limits the possibilities to optimize physical properties, such as the compatibility with different radiation curable compositions, to the choice of Q. Using part of the hydroxyl groups of the core to introduce moieties for optimization of physical properties would lead to multifunctional initiators with a low functionality. High concentrations of photoinitiators would then be needed to obtain the required curing sensitivity, thus limiting the possibilities for the composition and having a large influence on the properties of the composition and the final result.
WO 9717378 (COATES BROTHERS PLC) discloses a different type of multifunctional initiators obtained by the reaction of a multifunctional core material containing two or more reactive groups and a photoinitiator or derivative thereof. The photoinitiator or derivative thereof has a reactive group capable of reacting with the reactive groups of the multifunctional core. The photoinitiators disclosed in WO 9717378 (COATES BROTHERS PLC) are low molecular weight compounds, having a maximum functionality of 6. Depending on the functionality of the polyfunctional initiator, the molecular weight of the core is preferably less than 500 for a difunctional initiator, preferable less than 1000 for a tetrafunctional initiator and less than 1500 for a hexafunctional initiator. For ink-jet applications, a further increase of molecular weight would lead to an unacceptable viscosity of the radiation curable ink-jet ink.
WO 9749664 (LAMBSON FINE CHEMICALS) discloses a photoinitiator, comprising a photoreactive portion and a pendant group, the photoreactive portion including an aromatic moiety and the pending group incorporating at least one optionally substituted poly(alkylene glycol) moiety. Preferred photoreactive portions include optionally substituted benzophenone, thioxanthone and anthraquinone compounds substituted by a polyethylene glycol or polypropylene glycol moiety of an average molecular weight in the range 150 to 900. These types of initiators are essentially monofunctional. The molecular weight per photoreactive moiety is high. For an equal molar initiator concentration, a high weight percentage of these macromolecular initiators are required compared to their low molecular weight counterparts. As a result a high amount of unreactive polymer is introduced in the radiation curable formulation, having a negative influence on physical properties such as scratch resistance.
Especially in ink-jet applications, a significant increase in the solution viscosity has to be avoided to keep the ink-jet ink jettable. One approach to reduce problems of high viscosity and low functionality caused by these polymeric initiators and co-initiators is to combine the initiator and co-initiator in the same macromolecule.
Some combinations of a co-initiator and an initiator in a polymer have been described. Poly(ethylene imines) derivatized with thioxanthone moieties have been reported by Jiang et al. (Polymer, 45 (2004), 133-140).
In another approach, Jiang and Yin reported the polycondensation product of specific thioxanthone-derivatives with amines as the combination of Norrish type II photoinitiator and a synergist in the same polymer (Polymer, 45 (2004), 5057-5063).
Amine modified polyether imides have been reported by Davidson et al. (Journal of Photochemistry and Photobiology, A: Chemistry (1995), 91(2), 153-163), while Angiolini et al. reported acrylate copolymers having both an initiating moiety and a tertiary amine (benzoin methyl ethers and tertiary amines: Polymers for Advanced Technologies (1993), 4(6), 375-384; benzophenones and tertiary amines: New Polymeric Materials (1987), 1 (1), 63-83; camphorquinone and tertiary amines: Macromolecular Chemistry and Physics (2000), 201(18), 2646-2653; thioxanthones and α-morpholinoketones: Polymer (1995), 36(21), 4055-60).
Also in two recent reviews, polymers having both an initiating and a co-initiating moiety have been reported (Corrales et al. in Journal of Photochemistry and Photobiology, A: Chemistry (2003), 159(2), 103-114 and Carlini et al. in Polymers for Advanced Technologies (1996), 7(5 & 6), 379-384).
Although several of these polymers show interesting photochemical properties, all of them have a linear geometry. Using these photoreactive polymers, the solution viscosity still increases to an undesirable level for a great number of applications with radiation curable compositions, e.g. ink-jet inks and lacquers.
There is therefore a need to provide cheap, effective photoreactive polymers suitable for radiation curable compositions for use on food packaging with these photoreactive polymers not being extractable into food or adversely affecting the physical properties of the packaging material. The photoreactive polymers should be easy to manufacture and should be compatible with a wide range of radiation curable compositions without causing high solution viscosity.